In communication systems, modulation and demodulation involves changing a signal's amplitude, frequency, and/or phase. For the ease of characterizing these properties, as well as to facilitate the implementation of the modulation and demodulation circuitry, the signal is commonly expressed as a vector with rectangular coordinates of I (in-phase) and Q (quadrature). Thus, transceiver designs frequently involve ways to process the I and Q components separately.
Note that in the following discussion, the transmitters and receivers described can be both stand-alone components and transmitter/receiver portions of transceivers. FIG. 1 is a block diagram illustrating a receiver that processes I and Q signal components. A local oscillator (LO) 115 generates a radio frequency (RF) signal that runs at the carrier frequency. Mixer 105 mixes the RF signal with the receiver input, Rx, to produce the I component of the intermediate frequency (IF) signal. At the same time, phase shifter 110 shifts the phase of the RF signal by 90°. The phase shifted signal is mixed with the receiver input by mixer 100 to produce the Q component of the IF signal. ADCs 125 and 120 convert the I and Q components to digital, and the digitized signals are sent to DSP 130 for further processing.
In the receiver design disclosed above, two ADCs are needed to convert the two components of the IF signal. It is desirable to reduce the number of ADCs to achieve a simpler transmitter design that is cheaper to produce, consumes less power, and is smaller in area
In existing transmitter designs, the input of the transmitter comes from the output of an external source. The external source, such as a baseband modem, traditionally produces analog outputs. As a result, most of the transmitters are designed to do modulation and processing in analog domain. As digital modulation and demodulation techniques become more popular because of their robustness and flexibility, it is desirable to convert the analog inputs to digital, and then do the modulation and filtering in the digital domain. Furthermore, it is desirable to reduce the number of ADCs needed to convert the I and Q components of the input in the transmitter.